The task of an optical sighting device on, say, a rifle, is to form a virtual image of an internal reticle superimposed, in all dimensions, on the image of an external target, so as to display to the rifleman the actual aimed-at point on the target. There are two basic types of such a device. The first type is the well-known telescopic sight, in which the reticle is placed at the plane where the objective lens forms the image of the target. The second type is a beam splitter arrangement in which a lens device having a concave semi-silvered surface mirror is provided at a plane forming an angle with a sight line so that a slightly attenuated view of the target is transmitted through the mirror nearly undisturbed. At the same time, a collimated virtual image of an illuminated reticle is reflected by the lens device towards the viewer's eye, the illuminated reticle being located in or adjacent to the focal plane of the mirror surface of the lens device. Thus the mirror suface produces from the light from the reticle, a virtual image which is perceived by the eye of the marksman as a bright spot located far ahead of the lens device and essentially on the external target itself. Examples of such sighting devices are disclosed in U.S. Pat. Nos. 3,942,901 and 4,402,605. The present invention concerns an optical sighting device of this second type.
One great disadvantage of beam-splitter type sighting devices, (and telescopic sights for that matter), is that their optical elements are located inside a relatively long tubular housing mounted to the top of the weapon being aimed. Accordingly, these devices have a very narrow field of view in the order of only 15.degree. or so. As a practical matter, in order to see the target through the sight, the marksman must position his head relative to the rifle so that one eye is located very close to the rear end of the sighting device, his other eye being closed. It will be appreciated, then, that it takes a relatively long time to obtain a bead on a target in this fashion. Furthermore, the fact that the marksman must aim at the target with one eye closed makes it difficult to hold the target in view for any length of time and causes eye strain and fatigue.
Some standard sighting devices are also disadvantaged in that they suffer a certain amount of parallax. Accordingly, when the marksman moves his eye perpendicular to the sighting axis of the device, the illuminated reticle shifts relative to the target, making it very difficult to hit the aimed-at spot on the target.
Some conventional beam-splitter-type sights are also relatively complex and costly to manufacture and they are usually adapted to fit only one specific type of firearm. In other words, it is difficult to retrofit conventional sighting devices to weapons already in the field.
Many standard sights are difficult to use due to several factors. In some, the illuminated reticle has a fixed intensity so that when the target is in bright sunlight, the reticle spot is difficult to see or, when the target is in deep shade, the very bright-appearing spot obscures the aimed-at point on the target. In other sights of this type, it is difficult to adjust the device to compensate for windage and the like.